Microlithography apparatus are conventionally used to transfer a desired pattern, defined by a mask or reticle, to a sensitized substrate (i.e., a substrate surface coated with a suitable photoresist or other photosensitive resin). The mask is typically formed by vapor deposition of a film of chromium or other suitable metal on a glass or analogous transparent material. The mask is irradiated with actinic light; light passing through the mask "prints" the mask pattern on the sensitive substrate.
Generally, microlithography apparatus and methods include (1) projection exposure, wherein the image on the mask is printed on the sensitive substrate using a projection optical system; (2) contact-exposure, wherein the mask and sensitized substrate directly contact each other during exposure of the sensitive substrate with the mask pattern; and (3) proximity-exposure, wherein the mask pattern is transferred to the sensitive substrate by irradiating the mask while the mask and the sensitized substrate are separated slightly from each other.
FIG. 1 shows a conventional proximity-exposure apparatus. Illuminant light is produced by a light source 1 and condensed by an elliptical mirror 2 to form a light-source image. Light from the light-source image is reflected by a planar mirror 3, converted into a substantially collimated beam by a collimating lens 4, and is incident upon a fly-eye lens 5. The fly-eye lens 5 splits the incident collimated light beam into a number of wavefronts, thereby forming multiple light-source images at the exit side of the fly-eye lens. Light from the multiple light-source images passes through, e.g., an aperture stop S defining a usually circular aperture. The light is condensed by a condenser optical system comprising a single concave mirror 6 having a front focal point at the location of the multiple light-source images. Light reflected from the concave mirror 6 illuminates a mask 7 placed superposedly relative to the sensitized substrate 8 (also termed herein a "workpiece"). The workpiece 8 is separated from the mask 7 by a prescribed small distance termed herein the "standoff." The illuminant light passing through transparent regions of the mask 7 are incident on the workpiece 8, thereby "transferring" the pattern defined by the mask 7 to the workpiece 8.
In conventional proximity-type microlithography apparatus as summarized above, the illuminance at the mask 7 and the obtainable resolution at the workpiece 8 are determined by the mask-side numerical aperture (NA) of the condenser optical system 4. More specifically, where B is the luminance of the light source 1, NA is the mask-side numerical aperture of the condenser optical system 4, and T is the transmittance of the optical system, the illuminance E at the mask is given by the following: EQU E=.pi..multidot.B.multidot.NA.sup.2 .multidot.T (1)
In conventional proximity-type microlithography apparatus, the luminance B is fixed. The NA of the illumination striking the mask 7 is also constant and is not a function of the direction of propagation of the illumination light. The NA, however, is dictated by the resolution required for transferring the mask pattern to the sensitive substrate. Due to problems arising from diffraction, the NA must be small whenever a large resolving power is required. I.e., as the mask-side NA of the condenser optical system is increased, edge regions of pattern features of the mask 7 are transferred to the workpiece 8 with less definition, i.e., the resolution with which the mask pattern is transferred to the workpiece deteriorates (FIG. 2).
With conventional proximity microlithography apparatus, it has frequently not been possible to obtain a desired illuminance of the workpiece while also ensuring attainment of a required pattern-transfer resolution. The only way to increase the illuminance E at the mask 7 in order to improve throughput while maintaining resolution is to increase the luminance B of the light source 1. This requires a light source 1 having greater output intensity, which means greater size of and heat generation by the light source 1. This, in turn, results in unwanted increases in size and complexity of the lamp unit used as the light source.